1. Field of the Invention
This invention relates to Faraday rotators, and more particularly to water cooled Faraday rotator element.
2. Description of the Prior Art
In the optical arts, a well-known device is a Faraday rotator which comprises a material having the characteristic that, when placed in an axial magnetic field, it exhibits a different index of refraction for circularly polarized light having a direction of propagation aligned parallel with the magnetic field than for circularly polarized light having a direction of propogation antiparallel to the magnetic field. Such devices are most commonly used in optical isolators and circulators for separating the various stages of master oscillator-power amplifier systems; separating the transmission and reception channels in systems (such as radar and line scanners) using common optical components and for isolating stable lasers from feedback and backscattered radiation which causes frequency instability in laser oscillators.
An exemplary form of an isolator using a Faraday rotor is disclosed and claimed in U.S. Pat. No. 3,617,129, filed on Nov. 10, 1969 and entitled “Interferometric Optical Isolator.”, U.S. Pat. No. 6,785,037, Aug. 31, 2004 entitled Faraday rotator, Matsushita and U.S. Pat. No. 6,813,076, Nov. 2, 2004 entitled Faraday rotator, optical isolator, polarizer, and diamond-like carbon thin film, issued to Okubo. The systems disclosed therein, are incorporated herein by reference.
Higher power tubes—gyro TWT's and gyro klystrons—can generate average power levels of many 10's kilowatts and possibly 100 kw or better at millimeter wave frequencies. Proposed radars which use these tubes must necessarily use beam waveguide (BWG) or quasi-optical techniques. Unfortunately, the transmission type Faraday rotators which have been used with se BWG radars at lower power levels cannot accommodate more than a few kilowatts. A new design is required, and the present invention is intended to satisfy that need.
Higher power tubes—gyro TWT's and byro klystrons—can generate average power levels of many 10's of kilowatts and possibly 100 kw or better at millimeter wave frequencies. Proposed radars which use these tubes must necessarily use beam waveguide (BWG) or quasi-optical techniques. Unfortunately, the transmission type Faraday rotators which have been used with se BWG radars at lower power levels cannot accommodate more than a few kilowatts. A new design is required.
A Lincoln Laboratory group (Ref. 1 through 4) proposed a reflective F.R. in which the back surface of the ferrite disk is metalized. The incident beam is reflected into a 90° turn (the angle of incidence with the ferrite disk θi=45°) and the metalized surface is water cooled. Expected power levels are perhaps two orders of magnitude greater than the transmission type F.R. Unfortunately, matching requirements for θi=45° differ substantially for the two orthogonal polarizations i.e. the s and p polarizations. This distorts the electrical performance of the device. The polarization rotation fluctuates and cross-polarization components are introduced to the output signal. A single quarter wave matching plate performs poorly. Dual matching plates, which are difficult to implement (and cool) theoretically improve performance, but not to the level of the transmission type device.
We describe here a near normal incidence (θi<10°) reflective F.R. for which the distinctions between the s and p polarizations vanish as well as the associated distortions. The design combines the advantages of the transmission and θi=45° reflective F.R.'s and avoids their disadvantages. How do we achieve near normal incidence without interference? It's done with mirrors. Really!